I do not use iBooks however the only way I could get Welcome to iBooks screen back was to delete the "$HOME/Library/Containers/com.apple.iBooksX" folder. Note that in doing this is may remove access to preexisting iBooks content, I don't know for sure since I do not use iBooks. Proceed at your own risk! (And why I'm posting this as a comment, not an answer.)
– user3439894Aug 10 '16 at 13:52

@user3439894 worked, but only once! The next time I turned it on and opened the app it was not the welcome screen! None of the ways worked. I think I should find a better way by myself ;-)
– YJBJan 9 '17 at 3:13